> Polish notation is easy for both to parse; humans only struggle because they’re not so familiar with it

I think you're downplaying the significance of this. The lack of familiarity is exactly what I'd argue makes a huge difference in practice even if theoretically the way our brains parse things isn't that different. We spend so much time reading and writing words that it requires effort to learn how to parse each specific symbol-oriented thing we might want to learn how to read. To add to the parent comment's examples, I'll throw in Brainfuck, which is an extremely simple language for a machine to learn to parse that's literally named for how impenetrable it looks to people at first glance.

"Simple if I spend the time to learn it" is not the same as "simple without having to spend time to learn it", and for some things, the fact that the syntax essentially ignores some of the finer details is the main feature rather than a drawback. When everyone I work with can read and write markdown good enough for us not to have major issues, and junior engineers can get up to basically the same level of competence in it without needing a lot of hand holding, it's just not worth the effort for me to try to convince everyone to use RST even if it is better in theory. The total amount of time I've spent dealing with the minor annoyances in markdown in my life is less than the amount of time it would probably take me to convince even one of my coworkers that we should switch all of our READMEs to RST.